The present invention relates to optical communication systems and more particularly, to systems and methods for compensating for chromatic dispersion in WDM signals.
The explosion of communication services, ranging from video teleconferencing to electronic commerce has spawned a new era of personal and business interactions. As evident in the rapid growth of Internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth of communication networks. Further improvements in optical communications hold great promise to meet the demand for greater bandwidth.
Wavelength division multiplexing (WDM) technology permits the concurrent transmission of multiple channels over a common optical fiber, thus expanding available bandwidth and providing other advantages in implementation. WDM techniques are increasingly being used to more optimally exploit the data carrying capacity of optical fibers. Current trends in WDM technology include: closer spacing of wavelength components, increasing numbers of WDM channels, higher data rates being carried in each WDM channel, and longer purely optical links with no intermediate regeneration of the optical signal based on recovery of data. In bringing about these advances, one phenomenon that must be overcome is so-called chromatic dispersion wherein different spectral components of a given optical pulse propagate down the fiber at slightly different speeds causing blurring of data-carrying pulses and intersymbol interference.
Chromatic dispersion compensation is thus an important part of the design of an effective WDM system. Increasing data rates, which require narrower pulses that are more sensitive to dispersion, mean that chromatic dispersion must be compensated more precisely. Chromatic dispersion compensation techniques include the use of dispersion compensating fiber and dispersion compensating gratings (DCG's). With the advent of DWDM (dense wave division multiplexing) systems that incorporate purely optical links of length greater than 1000 km and have greater than 200 channels, at 10 Gb/s per channel or smaller numbers of channels at 40 Gb/s per channel, it is necessary to provide optimal chromatic dispersion compensation to each WDM channel.
The degree of chromatic dispersion compensation that is required will vary for each channel. When chromatic dispersion compensation is applied at an intermediate point along the link, it is typically applied to the composite WDM signal rather than to the individual channels to avoid the need for intermediate demultiplexing and remultiplexing. The dispersion compensation can thus be adjusted to be optimal or nearly optimal for channels in the center of the WDM grid, but many channels at the edges will require further dispersion compensation at the receiver site.
FIG. 1 depicts a prior art approach to end-of-line chromatic dispersion compensation. The WDM signal includes 200 channels spaced 25 GHz apart and thus has an overall spectral width of 40 nm. The other channels have already experienced adequate chromatic dispersion compensation but the channels at the grid edges require further compensation. In the scheme of FIG. 1, this compensation is applied on a per-channel basis with the degree of compensation tailored to the chromatic dispersion on each channel.
An optical amplifier 100 preamplifies all of the components of the WDM signal. A deinterleaver 102 divides the 200 channel 25 GHz grid into four 50 channel 100 GHz grids. Each 100 GHz grid is amplified by a separate optical amplifier 104. Each optical amplifier 104 may be an erbium-doped fiber amplifier (EDFA) or a wide band lumped Raman amplifier (LRA) having 40 nm bandwidth. After amplification each of the grids is passed to a demultiplexer 106 that separates the grid into individual channels. The channels requiring dispersion compensation are individually amplified by EDFAs 108 to compensate for the insertion loss introduced by dispersion compensating components. For each channel requiring chromatic dispersion compensation, there is a four port circulator 110 configured in combination with dispersion compensating gratings (DCGs) 112 such that optical energy at the selected wavelength reflects off the gratings and experience the required degree of dispersion compensation. These DGS's 112 are optimized for the degree of compensation required on each channel.
This approach, while providing very precise dispersion compensation on a per-channel basis, carries many drawbacks. In particular, with increasing numbers of WDM channels to be compensated, the component cost becomes prohibitive and the overall package volume also increases to an undesirable extent. What is needed are systems and methods for providing sufficient dispersion compensation to each channel in a WDM system while minimizing cost and volume.